This invention relates to a process of forming a tin oxide coating on a face of a glass substrate by contacting such face while it is at elevated temperature with a gaseous medium containing tin tetrachloride which undergoes chemical reaction and/or decomposition to form said tin oxide coating.
The vapour deposition of metal oxide films on various substrates has been described in literature, see e.g. United Kingdom patent specification No. 702,774 and "Mechanism of CVD Thin Film SnO.sub.2 Formation" by R. N. Ghoshtagore in Journal of the Electrochemical Society, January 1978, page 110. These publications disclose the formation of metal oxide coatings by bringing hydrolyzable volatile chloride vapours into contact with the surface to be coated at elevated temperature and in the presence of water vapour. According to United Kingdom specification No. 702,774 a solution (which may be an aqueous solution) of an electroconductive film-forming compound e.g. stannic chloride, is sprayed onto a glass substrate at a temperature above 400.degree. F. while water is present in the atmosphere adjacent the face being coated. The specification states that the presence of water in the atmosphere where the coating formation occurs promotes the speed of film formation but the specification stipulates that the water content of the atmosphere in which spraying takes place must in all cases be less than 0.01 pounds per pound of air. The article by R. N. Ghoshtagore in J. Electrochem. Soc. Jan. 1978 describes the vapour phase deposition of thin stannic oxide films on heated amorphous silicon dioxide substrates from a contacting mixture of stannic chloride vapour and water vapour in a carrier gas.
For various purposes it is necessary to form tin oxide coatings having very good optical properties and in particular a uniform light transmitting power, which implies a uniform structure of the coating. The difficulty of achieving coatings of high optical quality are very appreciable if the coatings have to be formed continuously on a travelling substrate such as a glass ribbon and tend to be greater as higher substrate speeds are encountered. In the flat glass manufacturing industry it is often of importance to form optical coatings on the flat glass during its continuous production and the tendency is for production speeds to increase. For example float glass is produced at ribbon speeds of at least several meters per minute and speeds up to 12 meters per minute or more are attained in some plants.
The teaching derivable from the prior publications on chemical vapour deposition techniques does not enable tin oxide coatings of high optical quality to be reliably formed at high deposition rates on a glass substrate. The formation of surface coatings by a chemical vapour deposition technique is influenced by numerous factors additional to the more obvious ones, namely the composition of the vapour and the nature of the vapour deposition reactions. Such other factors include the temperature conditions at the coating site, and the composition of the substrate face itself. Although the rate of deposition of tin oxide on a travelling substrate made of glass can be increased by performing the coating operation in a moist atmosphere, the moisture level has to be very restricted when following the teaching of the prior publications, otherwise the results are unsatisfactory. The coatings exhibit objectionable internal haze and indeed this was to be expected from the aforesaid United Kingdom specification 702,774 because the occurrance of haze is referred to in that specification as a reason why the amount of water vapour should be very restricted.